CS 1917 is one of the most common markings on repros, BUT yours uses a very slightly different type face, slightly different leather stock, than the most common repros. It is also marked in a different place than most of the reproductions I have seen.

Still, I've not seen an authentic WWI sling that didn't evince considerable verdigris on the cuprous parts, and it looks as if it has been artificially "aged". Unless of course someone spent a lot of time cleaning up legitimate age.

I think it's a very well-done repro. But that's not definitive.

Good part: if it is a repro, there's no reason not to use it on a rifle actually in use. I'd still use some conditioner/neatsfoot on it before doing so, though. The real ones dry out after 99 years, and quite notably, fake "aging" can also cause deterioration...so!